1. Field of the Invention
The present invention relates to visual displays, particularly to addressable, reusable, paper-like visual displays, and more particularly to gyricon or twisting-ball displays.
2. Discussion of Related Art
A gyricon display, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic balls, each of which can be selectively rotated to present a desired face to an observer. For example, a gyricon display can incorporate balls each having two distinct hemispheres, one black and the other white, with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the balls are electrically as well as optically anisotropic. The black-and-white balls are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities and is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accommodate the balls, one ball per cavity, so as to prevent the balls from migrating within the sheet. A ball can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either the black or the white hemisphere to an observer viewing the surface of the sheet. Thus, by application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the balls can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image.
An exemplary gyricon display 10 is shown in side view in FIG. 1. Bichromal balls 11 are disposed in an elastomer binder of the sheet 12 that is swelled by a dielectric fluid creating cavities 13 in which the balls 11 are free to rotate. The balls 11 are electrically dipolar in the presence of the fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes 14a, 14b. The electrode 14a closest to viewing surface 15 is preferably transparent. An observer at I sees an image formed by the black and white pattern of the balls 11 as rotated to expose their black or white faces (hemispheres) to the viewing surface 15 of sheet 12.
U.S. Pat. No. 5,389,945, incorporated by reference herein, shows that gyricon displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, not found in CRTs, LCDs, or other conventional display media. Gyricon displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays.
Typically, known gyricon displays are made up of bichromal balls that are black on one hemisphere and white on the other. Other kinds of balls are also known. For example, U.S. Pat. No. 4,261,653 shows a multilayer ball, although it is made at least in part from glass and its use depends on an addressing scheme involving high-frequency electric fields.
Other commonly owned patents related to gyricon displays, each incorporated herein by reference in their entireties, include U.S. Pat. Nos. 5,262,098, 5,344,594, 5,717,514, 5,815,306, 5,989,629 and 6,097,531.
One shortcoming of gyricon displays that still needs improvement is the fact that the bichromal balls typically suffer a fixed loss of their dipole moments shortly after the displays are made. This is not a continuous loss process, but rather a step loss. If the bichromal balls have a large dipole moment to begin with, this will not result in an appreciable loss in the brightness or contrast of the display. However, as is the case with bichromal balls made from many pigments, the dipole moments are not especially large and the step loss in their dipole moments results in severe deterioration of the display properties.
It is therefore an object of the present invention to develop a gyricon display that possesses a longer stable operating time without adversely affecting the quality of the images displayed.
This and other objects of the present invention are achieved herein by inclusion of a chemical absorbent in association with the gyricon sheet. The chemical absorbent may be mixed into the elastomer of the gyricon sheet itself, it may be included in a separate scavenger layer directly adjacent and in contact with the gyricon sheet or it may be placed adjacent to the gyricon sheet.
These and other objects are also achieved by methods of making the gyricon sheet and methods of packaging the gyricon sheet into a display, which methods include incorporating the chemical absorbent into or adjacent to the gyricon sheet.